This invention relates to printers. In particular, this invention relates to thermal transfer printers for the printing of labels.
In thermal transfer printing, a layer of print material, such as ink, is heated to be selectively applied to the print media (e.g., a label). Commonly, the ink is carried on a ribbon which, along with the label, is fed between a thermal print head and a platen roller. As the ribbon passes the thermal print head, the thermal print head melts the ink from the ribbon for transfer onto the label. Ideally, the thermal print head applies sufficient nip pressure against the platen roller to effectively transfer the ink from the ribbon onto the label. Additionally, the nip pressure between the thermal print head and the platen roller may help to maintain tension on the ribbon as it is fed past the print head.
In thermal transfer printing, the print quality depends on the evenness with which the print head applies pressure across the ribbon and the label. For example, if uneven pressure is applied by the print head across the ribbon, then some portions of the ribbon may heat quickly while other portions of the ribbon may heat slowly. On the portions of the ribbon that heat more slowly, the ink may not fully melt for transfer to the print media. Likewise, if the pressure applied across the ribbon and the label is uneven, then there may be portions of the interface between the ribbon and the label that are not pressed together. If there is poor contact between the ribbon and the print media, then the ink may not transfer from the ribbon to the label, resulting in poor print quality.
Further, it is often desirable that a printer be capable of printing on various widths of print media such that a single printer is capable of printing on a variety of print media. However, in thermal transfer printers, the accommodation of the various sizes of print media poses serious challenges, particularly when coupled with the aforementioned requirement of applying even pressure across the print head. In particular, when the print media is narrower than the width of the print head, a decision needs be made about the manner in which the print media will be justified in the printer.
To avoid some of the problems associated with uneven pressure across the print head, the print media could be center justified. However, center justification typically requires that there be two moveable edge guides.
Moveable edge guides (1) complicate the assembly of the printer and (2) require action by the user to make proper edge guide adjustments for each print job. Moreover, center justification can cause problems with the placement of label registration sensors within the printer that target the location of the print on the print media.
To avoid some of the problems associated with moveable edge guides and sensor placement, the print media may be edge justified to one side of the print head. Although edge justification of the media on the platen roller alleviates some of the problems with guiding and sensing, uneven print head pressure becomes a larger problem, particularly when printing onto print media that is less than half the width of the print head through the printer. As a result, the print on or near the outer edge of the print media may be light. Typically, an adjustment to a printing load mechanism is required to redistribute the pressure of the print head uniformly across the print media. However, this pressure skew adjustment is undesirable as it makes the printer more difficult or confusing to operate for the end-user. Alternate solutions to having the user make the pressure skew adjustment are possible, but are expensive to implement (i.e., the addition of sensors and pressure adjusting mechanisms).
Hence, there is a need for an improved print head that uniformly applies pressure across the platen roller, regardless of the justification of the print media. Moreover, there is a need for improving the print quality of thermal printers, particularly in edge-justified printing operations.